Quantitative Prediction of Adsorption and Diffusion in Pure Silica and Cationic Zeolites

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Findley, John
Sholl, David S.
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Zeolites are a class of nanoporous aluminosilicate materials. They are often used industrially for separations and catalysis because of their low cost and high thermal stability. The variety of exchangeable cations, Si/Al ratio and aluminum distribution can affect the adsorption and diffusion properties of these materials. Molecular simulations provide an inexpensive, well-defined way to study the effects of these properties on measurable quantities, such as adsorption and diffusion. In this work, we developed methods to examine the effects of aluminum distribution in zeolites and more accurate force fields for predicting adsorption and diffusion. We first examined the effect of aluminum distribution on CO2 adsorption in cationic zeolites. We observed a significant dependence of extra-framework cation distributions and CO2 adsorption properties on aluminum distribution. This indicated that aluminum ordering should be considered when screening cationic zeolites for CO2 adsorption and that CO2 adsorption isotherms can be used to probe aluminum distribution. Next, we developed accurate, transferable force field methods that are used to examine adsorption and diffusion in both pure-silica and cationic zeolites. In both cases, the force fields were fit to reproduce DFT/CC energies of both transition state configurations and energy minimum configurations to enable accurate predictions for both adsorption and diffusion data for a wide array of adsorbates in both pure-silica zeolites and cationic zeolites. Overall, in this work we developed more transferable tools for predicting both adsorption and diffusion in both pure-silica and cationic zeolites, which previous classical simulation methods were limited to predicting adsorption for pure-silica, Na-exchanged and K-exchanged zeolites.
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